This project concerns studies of Rocky Mountain spotted fever and other tick-borne rickettsial diseases in the United States and in certain other countries with emphasis on ecology, identification, and characterization of rickettsiae and their relationship(s) to the respective tick vectors. Source material is obtained through collaboration with outside agencies. The project also considers the cellular and subcellular aspects of interactions between tick-borne rickettsiae and their vectors, particularly the mechanism(s) of interference and the factors responsible for changes in the agent's pathogenicity. Isolation of rickettsiae are made from infected ticks or from blood of patients or animals by injection into susceptible animals or cell cultures. Characterization of isolates includes serological and biochemical methods (microagglutination, microimmunofluorescence, DNA base composition and protein (SDS-PAGE) determinations). Interactions between rickettsiae and their arthropod vectors is followed by light, fluorescence and electron microscopy of tissues from naturally or experimentally infected ticks. Tissue cultures are being used to study mechanisms of rickettsial development and infection in host cells. Rocky Mountain spotted fever and ehrlichiosis in dogs, although difficult to differentiate clinically, can readily be diagnosed by specific immunofluorescence. The newly detected etiologic agent of Potomac Horse fever is a rickettsia of the genus Ehrlichia. The name E. risticii nov. sp. has been proposed. Although the modes of transmission of this agent has not as yet been established, ticks are considered as possible vectors. Rhipicephalus sanguineus and Dermacentor variabilis have been shown to maintain this agent. Attempts to isolate R. rickettsii from D. variabilis collected in spotted fever areas of North Carolina and Indiana have so far been negative. The only rickettsial agent recovered is R. montana. Monoclonal antibodies to R. rickettsii are being evaluated for rapid identification of ticks infected with this agent.